[frtR Gene Affects Acid Production and Demineralization Ability of Streptococcus mutans].

Objective: To study the effect of the frtR gene of TetR family on the acid production ability of Streptococcus mutans( S. mutans) and the bacteria's ability to induce tooth demineralization . Methods: The growth of two strains of S. mutans UA159, Δ frtR, the frtR gene in-frame deletion strain, and Δ frtR/pDL278- frtR, the complement strain, was examined. The structure of biofilm was observed by laser scanning confocal microscopy (LSCM). The quantitative determination of water-insoluble extracellular polysaccharide (EPS) in the bacterial biofilms was done by anthrone-sulfuric acid method. The acid production capacity of S. mutans was measured by glycolytic pH drop. The demineralization-inducing ability of the strains on bovine teeth was determined by transverse microradiography (TMR). Results: The growth curves of the strains showed that frtR did not affect the growth of S. mutans. According to the findings of LSCM observation, frtR did not affect the biofilm formation. According to the findings of the anthrone-sulfuric acid method, frtR did not have any significant impact on the EPS synthesis of S. mutans. The results of the glycolytic pH drop assay showed that the deletion of frtR delayed the rate of acid production by S. mutans when sucrose was the only carbon source. In addition, according to the TMR results, knocking out frtR reduced the depth and amount of demineralization induced by S. mutans on the surface of bovine teeth. Conclusion: The deletion of frtR can weaken the acid production ability and the demineralization ability of S. mutans.

[Inhibitory Effects of Nicotinamide on Streptococcus mutans Growth and Biofilm Formation].

Objective: To explore the effects of nicotinamide (NAM) on the growth, biofilm formation and exopolysaccharides (EPS) production of Streptococcus mutans. Methods: The minimum inhibitory concentration (MIC) of NAM on S. mutanswas determined by the planktonic bacterial susceptibility assay. The NAM mass concentrations were set as 1/2 MIC, 1/4 MIC and 1/8 MIC for hree separate treatment groups. Culture medium without NAM was used in the negative control group and culture medium containing 0.1 mg/mL NaF was used for the positive control group (except for the scanning electron microscopy). The growth curves of S. mutans under different NAM concentrations were drawn. Crystal violet assay and anthrone-sulfuric acid method were used to explore the effects of NAM on S. mutans biofilm formation and water-insoluble EPS production, respectively. The morphology and structure of S. mutansplanktons and biofilms after NAM treatment were observed by scanning electron microscopy. Results: The MIC of NAM on S. mutans was 32 µg/µL. After 16 µg/µL (1/2 MIC), 8 µg/µL (1/4 MIC) and 4 µg/µL (1/8 MIC) NAM treatments, S. mutans growth and biofilm formation were inhibited, with the 16 µg/µL NAM group displaying the most significant inhibitory effects. The synthesis of EPS decreased significantly in the 16 µg/µL and 8 µg/µL NAM groups in comparison with that of the negative control group (P<0.05). Under scanning electron microscope, the cell length of S. mutans was shortened, the cell width was extended, and the length/width ratio was decreased, showing significant difference when comparing the 16 µg/µL and 8 µg/µL NAM groups with the negative control group (P<0.05). Conclusion: Under the influence of NAM at certain concenrations, the growth, biofilm formation, and EPS synthesis of S. mutanswere inhibited.